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400mm High 800mm Length T11 Style 2000 type Single Panel Radiator With Convector
Too big and the system will overshoot its temperature and be less economical to run, too small and it won't reach its desired temperature.
(Some of the poorer installers get round this complex step by putting in radiators that are to big, and then fitting thermostatic radiator valves to every radiator to cut the heat down.) This calculation produces a Heat loss figure in watts, of how much heat you need to warm that room up to the design temperature from -3 deg C in one hour,